Keep in mind that federal ownership doesn't mean that the land can't be used productively or generate revenue. Mineral rights, range leases, and many more things don't require selling the land. Having large regions close to their natural state is also a priceless treasure, and checker-boarding them, or ruining the view at national parks, is a real net negative. (That said, I do think there probably are federal lands it could make sense to sell. But I have no confidence coming up with an arbitrary amount of land to sell to raise revenue is a good policy.) Also keep in mind, that transforming some of these areas to residential use might have complicated impacts on water use and wetlands, though presumably less than agriculture would.